Isotropic absorption

RAIRS is a non-destructive infrared technique with special versatility - it does not require the vacuum conditions essential for electron spectroscopic methods and is, therefore, in principle, applicable to the study of growth processes [4.270]. By use of a polarization modulation technique surfaces in a gas phase can be investigated. Higher surface sensitivity is achieved by modulation of the polarization between s and p. This method can also be used to discriminate between anisotropic near-sur-face absorption and isotropic absorption in the gas phase [4.271]. 

For systems where the polarization dependence of the edge structure is already known, polarized measurements could be used to determine the sample orientation. For example, the intense pre-edge transition described for (acac) V 0 is also found in the isotropic absorption spectra of M 0(porphyrin) (M Ti,V,Cr) systems (33). Strongly polarized transitions like these could be used, for example, to determine the orientation of the porphyrin moiety within an ordered system such as a biological membrane or fiber (24). 

In /(,y-unsaturated ketones overlap of the olefinic rc-orbital with the orbitals of the carbonyl group leads to enhancement of both the isotropic absorption and the Cotton effect of the n-71 band. The generalized" octant rule predicts a large positive n-71 Cotton effect when the C = C bond in /j.. -imsatijrated ketones of cisoidgeometry appears in a positive octant [Figure 6 (a)]43. 

The unique spectral absorption of the Rhodonines contains two visual band components, an isotropic absorption associated with the conjugated dipole molecular structure of the molecule, and a anisotropic absorption associated with an additional resonant slow-wave stmcture intimately associated with the triplet electrons of the oxygen atoms of the molecule. The unusual relaxation properties of these molecules are also associated with these triplet state electrons. The Rhodonines do not fluoresce or phosphoresce significantly while in a dilute liquid solution. 

Molecular absorption-A higher energy-level change involving a change in the energy levels of a molecule and characterized by isotropic absorption in the visible or ultraviolet range. 

The unique absorption geometry of the conjugated molecules as a function of their state of matter is illustrated here. When in dilute solution, the molecules exhibit an isotropic absorption characteristic. 

Wald, et. al97,98. performed a set of experiments during the 1940 s that purported to demonstrate the formation of rhodopsin from either retinene, (now known as retinal) or Vitamin A, and a native protein. While their work involved materials showing a peak absorption at 500 nm, this is the wavelength of peak isotropic absorption of a large number of dipolar retinoids. Such a peak is not exclusive to the chromophoies of vision. Neither is it relevant to the anisotropic absorption spectrum of the chromophores of vision. 

For those retinoids rigidized by conjugation and containing two heavy atoms, the molecules may exhibit a significant absorption spectrum in the visible due to dipole-molecular excitation. This absorption spectrum, peaking at 502 nm, is shown by the solid black fine. It can be described in terms of a finite isotropic absorption cross section. 

When coated onto the disks of the Outer Segment, the materials show both an isotropic absorption spectrum due to... 

The spectral data in the literature agrees quite well with the spectral performance of the Rhodonines predicted by the theoretical part of this work. This is particularly true with regard to the data from the 1970 s forward. Of course, it requires that one segregate the isotropic absorption data from the anisotropic data. 

Baylor et. al. have provided recent in-vitro isotropic absorption spectrum for Rhodonine based on Macaca fascicularis143. The data was collected over five and one-half orders of intensityusing transverse illumination at near 37 Celsius. Baylor, et. al. also provide a review of other available measurements and perform a circuitous calculation to compare the isotropic spectrum to the human scotopic luminous efficiency function attributed to Carpenter rather than the C.I.E. Based on ten photoreceptors, the peak sensitivity was estimated to be 491 nm. This is lower than most other investigators. However, data points were only collected at 20 nm intervals using filters of 10 nm nominal half-widths. This sampling plan does not support three digit accuracy in their peak value. 

The results in their Table 1 show the exploratory nature of their experiments. The peak in the isotropic absorption of the mid wavelength photoreceptors of these species ranged from about 504-505 nm for the largely Vitamin A based animals to a peak near 514-523 (n=l 1) for the presumed Vitamin A2 based animals. This is the largest difference based on Vitamin A type found in the literature. 

Figure 5.5.10-5 CR Absorption spectrum of putative frog rhodopsin as a function of bleaching level. Curve 1 is unbleached. The material was prepared as a red powder and then placed in dilute solution. From Wolken, 1966. (A) Intrinsic ultraviolet peak of a retinoid. (B) intrinsic isotropic absorption peak of a retinol (al) complexed with opsin. From wolken, 1966.

It is significant to note the shape of the anisotropic spectrum of the ultraviolet absorber, Rhodonine (11) measured by these workers, does not correspond to the isotropic absorption characteristic of the dilute retinoids. The anisotropic spectrum reflects resonant conjugate absorption in the Rhodonines. Note also the lack of any absorption characteristic with a peak near 502 nm. 

Like UV absorption and natural CD spectra, the MCD spectrum observed in the UV/VIS region is a superposition of contributions from all electronic transitions in the molecule. Whereas in ordinary isotropic absorption spectra or in natural CD spectra each transition can be characterized by its band shape and a single scalar quantity, the oscillator strength or the rotatory strength, three numbers, A Bj, and C are necessary to describe the contribution of each electronic transition i to the MCD spectrum of an isotropic sample. The three numbers are known as the A term, the B term, and the C term of the i-th transition. The contributions of the B term and of the C term to the MCD curve [0]m(v) have the shape of an absorption peak, which in the simplest case has a Gaussian profile as shown in Figure 3.8. They differ insofar as the contribution of the B term is temperature independent, whereas that of the C term is inversely proportional to the absolute temperature and can therefore dominate the spectra at very low temperatures. If nonzero B and C terms are both present, a measurement of the temperature dependence of the spectrum is needed to separate them. 

CD spectra carry much more information than do UV spectra the intensity of the CD absorption is dependent upon the spatial relationship between the chromo-phore and groupings at the chiral centre and therefore there is no chromo-phore-intensity-of-absorption relationship such as that which exists for UV spectra (i.e. the Bouguer-Bccr Lambert law does not apply to CD spectra). Also, the sign of the CD feature can be positive or negative, unlike the isotropic absorption (i.e. the UV spectrum), which has no sign. 

B -t C/kT), and the area of the corresponding isotropic absorption provides the associated dipole strength D. For the A2 -> Ti (F) d—d transition of tetrahedral cobalt(II) complexes, the ratio of the two quantities derived directly from the MCD and absorption band areas is given by Eq. (18),... 

For example, most prototype devices use polymethyl methacrylate CPMMA, trade name PLEXIGLASS) as a substrate material. PMMA has an index of refraction of about 1,49, This produces an escape probability of 26% for isotropic absorption and emission, and an escape probability of 29% for dipole absorption and emission of perpendicularly incident light. 

Table 3.6. Isotropic absorption indexes (/fiao), FWHM, and TDM orientations for CH stretching bands of dioctadecyl (C18H37) group of dioctadecyl disulfide ...

Fig. 3. Isotropic (dashed line) and linear dichroism (full line) spectra of 2,4,6-trimethylpyridine-N-oxide in the 260-350 nm region. The isotropic absorption was measured at 345 K. The optical density (OD) and the optical differences (AOD) are quoted to the right and to the left of the figure, respectively. Cell length 0.1 inm, cone. 5xl0 (mol dm ),...

Fig. 4. Linear dichroism (top) and isotropic absorption (lower) spectra of 9,10-diazaphenantrene.

The efficiency of PL polarizers is chiefly limited by the luminophore s quantum yield which, ultimately, can approach unity, but for PL polymers typically is up to 80 % (24,33). However, when used in a standard PL LCD configuration (see above), only 50 % of light incident from the light source is used, since the absorption of these PL polarizers is also anisotropic (74-25). We recently have demonstrated a new concept for polymer-based PL polarizers which overcome this limitation and can be used in PL LCDs with, in principle, an ultimate efficiency of unity (34,35). These PL polarizers comprise a nearly randomly oriented sensitizer that maximally harvests light by isotropic absorption, efficiently transfers the energy to a uniaxially oriented PL polymer which, subsequently, emits highly linearly polarized light. Key step is a... 

With the formalism of the time-dependent perturbation theory for interaction between an electron and the classical radiation field, within the electric dipole and Bom-Oppenheimer approximations, the isotropic absorption cross section is given by [17]... 

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